There is substantial evidence that arginine vasopressin (AVP) is a peptide transmitter in the rat hippocampus. It is present in hippocampal nerve fibers and released upon depolarization. Specific receptors exist for the peptide in hippocampal membranes and AVP has action on neural targets in the hippocampus that is blocked by a specific structural antagonist. the peptide has behavioral effects that are likely mediated by hippocampal structures. The only obstacle to understanding the brain AVP system is lack of control over endogenously released AVP. Recently, the source of the AVP projection to the hippocampus has been identified. It is not one of the major peptidergic nuclei of the hypothalamus, but rather it is a discrete cell population in the medial amygdala. here i propose to stimulate this nucleus in the whole animal and record the peptidergic, AVP-mediated response in the hippocampus. An effective, structural antagonist for AVP will be used to block the response, demonstrating specificity and AVP mediation of the signal. The experiments will open up additional lines of investigation leading to the elucidation of the role of endogenous AVP in the generation behavior.